1. Technical Field
Embodiments of the subject matter described herein relate to an exhaust gas recirculation (EGR) system, a cooler for that system, and associated methods.
2. Discussion of Art
Engines may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system, a process referred to as exhaust gas recirculation (EGR). In some examples, a group of one or more cylinders may have an exhaust manifold that is coupled to an intake passage of the engine such that the group of cylinders is dedicated, at least under some conditions, to generating exhaust gas for EGR. Such cylinders may be referred to as “donor cylinders.” In other systems, the exhaust gas may be pulled from a manifold.
Some EGR systems may include an EGR cooler to reduce a temperature of the recirculated exhaust gas before it enters the intake passage. The exhaust gas recirculation (EGR) cooler may be used to reduce exhaust gas temperature from about 1000 degrees Fahrenheit to about 200 degrees Fahrenheit. In such an example, fouling of the EGR cooler may occur when particulate matter (e.g., soot, hydrocarbons, oil, fuel, rust, ash, mineral deposits, and the like) in the exhaust gas accumulates within the EGR cooler. The EGR cooler can foul over time due to various factors (duty cycle, time at idle, engine oil carryover, time in service) decreasing effectiveness of the EGR cooler and increasing a pressure drop across the EGR cooler as well as temperature of the gas exiting the cooler. This could result in increased level of emissions and decreased fuel efficiency.
Some EGR coolers may fail during use due to high stress concentration in tubes at a leading edge of the heat exchanger—the edge that is closest to a tube sheet. The proximity would sometimes subject portions of the system to high stress due to low water flow, over constraint by a heat exchanger sidewall, and high thermal gradients.
If fouling occurs, the engine system switches into a cleaning mode referred to as port heating. Port heating is an operating mode that reduces an amount of (i.e. oxidizes and/or vaporizes) liquid oil that may be present (fouling) an exhaust system. In one example, during the port heating mode the system over-fuels individual cylinder(s) during engine idle. This over-fueling continues and heats the local exhaust port. The system engages port heating periodically at low loads, such as idle and/or in response to the engine experiencing conditions that put engine at risk for oil in the exhaust system. Fouling, or “souping,” can cause unburned oil to foul engine hardware such as the EGR cooler. If this unburned oil is blown out the exhaust stack, it may leave an unsightly residue on the exterior of the equipment and/or vehicle. Thus, port heating has been used to reduce oil residue fouling of the EGR cooler, engine intake, and equipment exterior.
It may be desirable to have an EGR cooler system that prevents fouling, or if fouled is easier to clean, than those systems that are currently available.